Magnesium base alloys



Sept. 30, 1969 w. H. o. ZIEGLER ET AL 3,

MAGNESIUM BASE ALLOYS Filed Sept. 12. 1967 INCIDENCE OF NETWORKS OF SECOND PHASE IN MqZn Mn ALLOY CAST BLOCKS X PRESENT OR INCIPIENT O ABSENT 5 X a? X X WT. "/0 MANGANESE United States Patent 3,469,974 MAGNESIUM BASE ALLOYS Wolfram H. 0. Ziegler, Uberlingen (Bodensee), Germany, and Philip A. Fisher, Swintou, England, assignors to Magnesium Elektron Limited, Swinton, Manchester, England, a company of the United Kingdom Continuation-impart of application Ser. No. 477,193, Aug. 4, 1965. This application Sept. 12, 1967, Ser. No. 667,165 Claims priority, application Great Britain, Aug. 7, 1964, 32,303/ 64 Int. Cl. C22c 23/00 US. Cl. 75-168 7 Claims ABSTRACT OF THE DISCLOSURE A magnesium base alloy in wrought condition which, apart from impurities, consists essentially of 06-13% by weight of manganese, 12-25% by weight of zinc and the remainder magnesium.

CROSS-REFERENCE This invention is a continuation-in-part of application Ser. No. 477,193, filed Aug. 4, 1965, now abandoned.

BACKGROUND AND OBJECTS'OF THE INVENTION This invention relates to the production of magnesium base alloys for use in the wrought condition produced by plastic deformation, viz. extruded, rolled or forged, and to ingots (i.e. billets or slabs) suitable for plastic deformation.

The use of magnesium base alloys for many applications is at present restricted for wrought applications because of the comparatively high cost of such alloys, the cost of production being unfavorably influenced by such factors as the relatively slow speed at which the alloys can be plastically deformed, the necessity to provide a smooth machined surface on the cast block in order to prevent cracking during deformation, and the high cost of certain alloying ingredients. Thus, for example, a well known magnesium base alloy contains nominally 3% aluminum, 1% zinc and 0.2% manganese and is sold under the designation AZ31. This alloy is presently used for the majority of wrought magnesium alloy applications. With this alloy, however, it is necessary when making extrusions that the cast billet be machined to remove the rough cast surface together with areas of higher alloy content which arise from segregation of aluminum and zinc to the billet surface. If such machining is not carried out there is a risk that the billet will oxidize during preheating for extrusion and that the rough cast surface will promote cracking in the surface of the extrusion. The temperature and speed of working AZ31 alloy must be restricted in order to prevent hot tearing. When this alloy is used in the form of welded sheet or plate it is necessary to give a stress relieving heat treatment to the welded structure to avoid the risk of stress corrosion in such a structure. To overcome such disadvantages it has been proposed to use an alloy containing nominally 1% zinc and rare earth metals but such an alloy has the disadvantage that the addition of rare earth metals is comparatively expensive, and its strength (maximum tensile stress and proof stress), is lower than that of AZ31.

Other prior art compositions proposed will not yield the extrusion speed and strength of the instant invention. These prior compositions (as exemplified by Gann US. Patent No. 1,886,251 and Italian Patent No. 403,750) must sacrifice either extrusion speed or strength or both.

Prior art attempts have shown a high incidence of networks of low melting point second phase which is known to cause possible hot cracking in working. Also, even at the low end of the ma-ganese range in the attempts illustrated, for example, in the Italian patent a high incidence of course manganese particles is obtained, thus causing difficulty in machining.

The present invention is based on the discovery that a narrow range of composition of one type of alloy using inexpensive ingredients can olfer mechanical properties as good as or even better than AZ31 while permitting an extrusion speed of at least double (and in many cases four times) the permissible speed of AZ31 with good surface appearance, corresponding high reductions in rolling accompanied by other exceptional advantages. For example, the alloy is weldable without requiring stress relief treatment. Cast ingots of the alloy of this invention have such a satisfactory even distribution of alloying components unimpaired by surface segregation that they can be extruded without first being required to be machined and, moreover, for example at temperatures of 450 C. in comparison with a maximum of 400 C. for AZ'31. This enables the pressure to be reduced for a given speed. Another extremely important advantage of the alloy of the present invention is that when cast ingots having some pores or voids are extruded normally at high speed the pores or voids are welded over and sound extrusions result.

It has been mentioned above that the temperature and speed of working of A231 alloy must be restricted. This is due to the presence of networks of a low melting point second phase which melts giving hot tearing at higher temperature or speeds. In the alloy of the present invention it has been found that the presence of second phase is suppressed within a narrow range of composition as shown in the accompanying drawing. The presence of networks of second phase is well known in the binary magesium-zinc alloys. It is surprising that the addition of manganese suppresses its incidence; it is even more surprising that with a further manganese addition it recurs. The accompanying drawing shows a graph illustrating the compositions of alloys in which the harmful networks of second phase occur and the compositions in wich these networks are absent. For the purpose of the preesnt invention alloys are used having a composition falling within the area below the graph curve. It is undesirable to use more than 2.5% zinc to ensure absence of low melting point second phase. The alloy should include at least 1.2% zinc for adequate strength. Also at least 0.6% manganese is included to achieve adequate strength but the manganese should not exceed 1.3% as above this there is a tendency to produce manganese particles in the alloy which are harmful when machining the alloy.

SUMMARY OF THE INVENTION According to the present invention the alloy consists apart from impurities of:

Manganese 0.6% to 1.3% preferably 0.8 to 1.1%. Zinc 1.2 to 2.5% preferably 1.7 to 2.2%. Magnesium Remainder.

and having a composition such as to be below the curve illustrated in the accompanying drawing.

Impurities may be included (e.g. iron and silicon) up to a total of 0.7%. Preferably the silicon will not exceed 0.02%. Aluminum in this alloy is regarded as an impurity and should be less than 0.5%. Beryllium is preferably omitted and is also to be regarded as an impurity.

DETAILED DESCRIPTION We have found it possible to produce extrusions of satisfactory quality without the necessity to machine the surface of the cast billet using extrusion speeds up to four times faster than those possible with AZ31 alloy, while still retaining both excellent surface quality in the extruded parts and also tensile properties comparable with those of AZ31 alloy, as illustrated by the following 5 bility of manganese in magnesium it is frequently the case that in an alloy such as AZ31 part of the manganese is present as segregations of manganese-rich particles. Such particles are comparatively hard and brittle and give rise to dithculty in application involving maresults: chining of the magnesium alloy. A further advantage of TABLE 1.TENSILE PROPERTIES Extrusion Extrusion 0.17 proof S eed, temperastress, U.T.S., Elongahon, Alloy ft. min. ture, C. t.s.i. t.s.1. percent Mg-27 Zn 0.87 Mn (in a tcordancf with :figf 11mm 45 450 11 16 16 Aza1 20 390 11 17 12 The high speed of extrusion possible with the alloy of the present invention is further illustrated for a hollow extrusion in Table 2.

TABLE 2.BILLE'IS 3' DIA. x 6' LONG EXTRUDED ON A 450 TON PRESS INTO TUBE O.D., 4' LD.

o. 530 Billet caught fire before extrusion. 480 Extrusion hot cracked and caught fire. 450 Billet too hard to extrude.

It will be seen that for this extrusion a speed of 30 ft-/ min. was too fast for AZ31. In fact, the A231 alloy would not extrude at the normal operating temperature for this alloy and raising the temperature merely resulted in hot tearing due to melting of the second phase present. On the other hand, the alloy of the present invention was easily extruded at fast speeds.

As a further example of the fast extrusion speed made possible by the alloy of the present invention strip of 1.5 inch width and 0.60 thickness could be extruded in A231 alloy at a maximum speed of 50 ft./min. whereas the alloy of the present invention was readily extruded at 200 ft./min., this speed being limited by the power of the extrusion press. To further illustrate these fast extrusion properties it was found possible to impact extrude a similar section at 400 ft./n1in.

It has also been found possible to use a wide range of temperatures for deformation of the magnesium-zinc-manganeSe alloys of the present invention. Thus extrustions have been successfully produced at temperatures of 350 to 530 C. and the alloy has been successfully rolled using temperatures at a commencement of rolling between 450 and 510 C. This compares, for example, with a normal temperature range for AZ31 of 350 to 400 C. for extrusion and 350 to 450 C. for rolling and forging.

Magnesium-zinc-manganese alloys as claimed in this invention have been found to have excellent corrosion resistance, being superior in this respect to magnesium-aluminium-zinc-manganese alloys with normal impurity contents.

For rolling and forging it has been found not only possible to commence plastic deformation at a higher temperature than heretofore but also to effect plastic defor mation at lower temperatures, e.g. down to 180 C. for rolling and 230 C. for forging. That is to say, the alloys of the present invention have a much Wider temperature range for plastic deformation than AZ31 alloy.

A further disadvantage of AZ31 is the occasional pres ence of manganese-rich particles. Manganese is added to this alloy to improve corrosion resistance but since the addition of aluminium to magnesium reduces the soluthe magnesium-zinc-manganese alloys covered by this invention is that although the manganese content is relatively high, since zinc does not suppress the solubility of manganese as does aluminium, the alloys are not subject to segregation of manganese-rich particles.

The alloys of the present invention may be prepared by adding manganese in the form of manganous chloride or as a magnesium-manganese pre-alloy or as metallic manganese added direct to the magnesium base alloy. As alternative method of preparation which provides for improved efliciency in manganese introduction is first to make a zinc-manganese hardener, e.g. approximately 50% zinc, 50% manganese or approximately 70% zinc and 30% manganese, such hardener subsequently being alloyed to the magnesium.

In casting the magnesium-zinc-manganese alloy with chill cast ingot it was found that the ingots had a grain structure comprising columnar crystals together with some equiaxed crystals and that the iron content was generally from 0.01 to 0.03 percent. For some purposes it may be desired that the grain structure in the cast ingot should be entirely composed of equiaxed crystals, and for some purpose it may be desired that the iron content be as low as possible.

It has been found that by adding a small quantity of zirconium to such an alloy the grain structure was refined to consist entirely of equiaxed crystals and that the iron content was reduced from about 0.03 percent to below 0.01 percent (e.g. to 0.005% the alloy also containing zirconium less than 0.1 percent, e.g. 0.02 percent). The zirconium content may be from 0.001 to 0.15 percent. The higher percentage of zirconium within this range is beneficial to grain refinement.

Alternatively, grain refinement may be effected by addition of iron, e.g. as ferric chloride or zinc-iron hardener alloy in which case iron contents up to 0.1%, generally 0.040.08%, are obtained. In this way fine grained structures (average grain dia. 0.1 mm.) are obtained in semicontinuously cast billet and slab. It has been confirmed that traces of silicon interfere with grain refinement, as illustrated by the results given in Table 3, and for this reason it is preferred that the silicon content of the alloy does not exceed 0.02%

TABLE 3.EFFECT OF SILICON ON THE GRAIN SIZE OF CAST Mg 2%, Zn 1%, Mn ALLOY TREATED WITH A FIXED ADDITION OF ZINC-IRON HARDENER ALLOY Silicon content, percent 005 0. 01 Grain size (mm. 0. 1 0.13

The use of iron and zirconium for effecting grain refinement may be further illustrated by the following examples:

Example 1.Zirconium grain refinement The magnesium was melted and the zinc added at 720 C. The manganese in the form of thin flakes was plunged into the melt at 760 C. The zirconium was added at 4 lb. of magnesium-zirconium hardener alloy (as advocated by British Patent No. 857,709) and plunged into the melt.

Final analysis: Zn 2.0%, Mn 1.1%, Zr 0.003%, Si

0.001%, Fe 0.006% Grain size: 0.2 mm.

Example 2.-Iron grain refinement Charge: Pounds Magnesium produced by thermal reduction with about 0.006% iron 255 Pre-alloy with 1.5% manganese 533 Zinc with 5% iron 18 The magnesium and the pre alloy with manganese were melted and the temperature raised to 750 C. The zinciron alloy was then stirred into the melt.

Final analysis: Zn 2.0%, Mn 1.0%, Fe 0.04%, Si 0.01% Grain size: 0.1 mm.

TENSILE PROPERTIES OF EXTRUDED Mg 2%, Zn 1%, Mn ALLOY 0.1% P. '1. S. Elongation s.

Condition t. s. i. 1.) (percent) As extruded 10. 3 16. 5 17 Heat treated 16 hrs. at 180 0 11. 6 17. 7 15 The graph clearly illustrates the incidence of networks of second phase in the Mg-Zn-Mn alloy cast blocks wherein the networks are present or incipient outside the curve and absent inside the curve. The presence of networks is indicated by X and the absence by 0. This clearly and graphically illustrates the advantages of instant invention.

While the invention has been described, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the essential features hereinbefore set forth and as fall within the scope of the invention or the limits of the appended claims.

We claim:

1. A magnesium base alloy capable of being plastically deformed over a relatively wide temperature range and at relatively high speeds without hot tearing, said alloy consisting essentially apart from impurities of:

(a) 0.61.3% by weight of manganese,

(b) 1.22.5 by weight of zinc, and

(c) the remainder magnesium. said alloy being characterized by the absence of networks of low melting point second phase.

2. An alloy as defined in claim 1 consisting essentially apart from impurities of:

(a) 0.8-1.1% by weight of manganese,

(b) 1.22.5% by weight of zinc, and

(c) the remainder magnesium.

3. An alloy as defined in claim 1 wherein the impurity silicon content is not more than 0.02% by weight.

4. An alloy as defined in claim 1 wherein the impurity iron content is at least 0.04% by weight.

5. An alloy as defined in claim 1 wherein the impurity iron content is not more than 0.005% by weight.

6. An alloy as defined in claim 5 including not more than 0.15% of zirconium by weight.

7. A wrought product made from the alloy of claim 1.

References Cited UNITED STATES PATENTS 1,886,251 11/1932 Gann 168 2,829,973 4/1958 Jessup et al. 75-168 3,067,028 12/1962 Foerster 75-168 FOREIGN PATENTS 403,750 Italy.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 14832, 32.5 

